The invention is in the field of determination of petrophysical properties of subsurface formations using data from a Nuclear Magnetic Resonance (NMR) tool. Specifically, the invention relates to the use of shaped pulses for reducing interference of signals from different regions of the subsurface in a gradient NMR tool using multiple frequency measurements.
A variety of techniques have been utilized in determining the presence and in estimating quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, porosity, fluid content, and permeability of the rock formation surrounding the wellbore drilled for recovering hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) or logging-while-drilling (xe2x80x9cLWDxe2x80x9d). Measurements have also been made when tripping a drillstring out of a wellbore: this is called measurement-while-tripping (xe2x80x9cMWTxe2x80x9d).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as xe2x80x9cT1xe2x80x9d), and transverse relaxation time (generally referred to as xe2x80x9cT2xe2x80x9d) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A typical NMR tool generates a static magnetic field B0 in the vicinity of the wellbore, and an oscillating field B1 in a direction perpendicular to B0. This oscillating field is usually applied in the form of short duration pulses. The purpose of the B0 field is to polarize the magnetic moments of nuclei parallel to the static field and the purpose of the B1 field is to rotate the magnetic moments by an angle xcex8 controlled by the width tp and the amplitude B1 of the oscillating pulse. With the variation of the number of pulses, pulse duration, and pulse intervals, various pulse sequences can be designed to manipulate the magnetic moment, so that different aspects of the NMR properties can be obtained. For NMR logging, the most common sequence is the Carr-Purcell-Meiboom-Gill (xe2x80x9cCPMGxe2x80x9d) sequence that can be expressed as
xe2x80x83TWxe2x88x9290xe2x88x92(txe2x88x92180xe2x88x92txe2x88x92echo)nxe2x80x83xe2x80x83(1)
where TW is a wait time, 90 is a 90 degree tipping pulse, 180 and is a 180 degree refocusing pulse.
After being tipped by 90xc2x0, the magnetic moment precesses around the static field at a particular frequency known as the Larmor frequency xcfx890, given by xcfx890=xcex3B0, where B0 is the field strength of the static magnetic field and xcex3 is the gyromagnetic ratio. At the same time, the magnetic moments return to the equilibrium direction (i.e., aligned with the static field) according to a decay time known as the xe2x80x9cspin-lattice relaxation timexe2x80x9d or T1. Inhomogeneities of the B0 field result in dephasing of the magnetic moments and to remedy this, a 180xc2x0 pulse is included in the sequence to refocus the magnetic moments. This gives a sequence of n echo signals.
U.S. Pat. No. 5,023,551 issued to Kleinberg discloses an NMR pulse sequence that has an NMR pulse sequence for use in the borehole environment which combines a modified inversion recovery (FIR) pulse sequence with a series of more than two, and typically hundreds, of CPMG pulses according to
[Wixe2x88x92180xe2x88x92TWixe2x88x9290xe2x88x92(xcfx84xe2x88x92180xe2x88x92xcfx84xe2x88x92echo)j]ixe2x80x83xe2x80x83(2)
where 90 is a 90 degree tipping pulse, 180 is a 180 degree refocusing pulse, j=xe2x88x921,2, . . . J and J is the number of echoes collected in a single Carr-Purcell-Meiboom-Gill (CPMG) sequence, where i=1, . . . I and I is the number of waiting times used in the pulse sequence, where Wi are the recovery times, TWi are the wait times before a CPMG sequence, and where xcfx84 is the spacing between the alternating 180xc2x0 pulses and the echo signals. Although a conceptually valid approach for obtaining T1 information, this method is extremely difficult to implement in wireline, MWD, LWD or MWT applications because of the long wait time that is required to acquire data with the different TWS.
There is an inherent inefficiency associated with the tipping and refocusing pulses in the CPMG sequence. The 90xc2x0 tipping pulse used to modulate the RF signal has a duration one half of the duration of the 180xc2x0 refocusing pulse, and, as would be known to those versed in the art, the shorter duration pulse has a large bandwidth than the longer duration refocusing pulse. Accordingly, only a portion of the pulses that are tipped will be refocused by the refocusing pulse.
In the typical NMR well logging procedure only about 5 to 10 percent of the total amount of time in between each NMR measurement set is used for RF power transmission of the CPMG pulse sequence. The remaining 90 to 95 percent of the time is used for repolarizing the earth formations along the static magnetic field. Further, more than half of the total amount of time within any of the CPMG sequences actually takes place between individual RF pulses, rather than during actual transmission of RF power.
Several methods are known in the art for dealing with the problem of non-transmitting time in an NMR measurement set. The first method assumes a known, fixed relationship between T1 and T2, as suggested for example, in Processing of Data from an NMR Logging Tool, R. Freedman et al, Society of Petroleum Engineers paper no. 30560 (1995). Based on the assumption of a fixed relationship between T1 and T2, the waiting (repolarization) time between individual CPMG measurement sequences is shortened and the measurement results are adjusted using the values of T2 measured during the CPMG sequences. Disadvantages of this method are described, for example in, Selection of Optimal Acquisition Parameters for MRIL Logs, R. Akkurt et al, The Log Analyst, vol. 36, no. 6, pp. 43-52 (1996). These disadvantages can be summarized as follows. First, the relationship between T1 and T2 is not a fixed one, and in fact can vary over a wide range, making any adjustment to the purported T1 measurement based on the T2 measurements inaccurate at best. Second, in porous media T1 and T2 are distributions rather than single values. It has proven difficult to xe2x80x9cadjustxe2x80x9d T1 distributions based on distributions of T2 values.
U.S. Pat. No. 6,049,205 to Taicher et al having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method for determination of T1 and T2. The static magnetic field in the disclosed device has a field gradient, so that an RF pulse of a selected frequency excites nuclei in a specific portion of the formation determined by the gyromagnetic ratio. By altering the frequency of excitation, different regions of the formation may be analyzed. U.S. Pat. No. 5,936,405 to Prammer et al teaches making interleaved measurements at different frequencies to obtain, in a single logging pass, multiple data streams corresponding to different recovery times and/or diffusivity for the same spot in the formation. The resultant data streams are processed to determine mineralogy-independent water and hydrocarbon saturations and porosity estimates.
In order for these multiple frequency measurements to be made efficiently, it is important that there be no interference between echo signals produced by the RF pulse sequences associated with the different frequencies of acquisition. As noted above, the bandwidth of the tipping pulse used in a CPMG sequence is greater than the bandwidth of the refocusing pulses. As a result of this, there is a possibility that a refocusing pulse associated with one of he CPMG sequences and having one RF frequency may have a sensitive zone in the formation that is affected by a tipping pulse of one of the other CPMG sequences. The interference between the wave trains can lead to erroneous results. To reduce interference between trains measured at neighboring frequencies f1 and f2, the separation between the frequencies has to be as large as possible. On the other hand, the electronics required for the wider separation is harder to build and the resulting pulse sequence is inefficient. It is desirable to have a method for multiple frequency NMR measurements that is relatively easy to apply and does not lead to a significant increase in the inefficiency of the logging. The present invention satisfies this need.
The present invention is a method for reducing the interference between echo trains of multiple frequency NMR logging applications using a gradient logging tool. The reduction in interference is obtained by shaping the envelope of the tipping pulse so that its bandwidth is reduced relative to a rectangular pulse producing substantially the same rotation of nuclear spins. The major contribution to the interference between different logging sequences comes from the sidelobes in the spectra of the excitation signal caused by modulating the RF signal with a square wave. In the present invention, by shaping the tipping pulse, the interference is greatly reduced. In an alternate embodiment of the invention, the refocusing pulses are also shaped.